How do underwater rebreathers scrub CO₂ from air supplies?

I know that alkali metal hydroxides (column I and II metal hydroxides) can be used to absorb carbon dioxide (CO₂) in an enclosed atmosphere, like in a space craft or submarine. However, they're corrosive and absorb water vapor. (1) Is there any commonly-available material that is not toxic/dangerous that will absorb CO₂? (2) What is used in underwater rebreathers/CO₂-scrubbers? Thanks.
Garcia Oliver

Plants absorb CO₂, of course, but they take up a lot of room and are slow, inefficient CO₂ absorbers.

Most industrial CO₂ scrubbers use chemicals that don't meet your criteria. Monoethanolamine (MEA) is used to scrub carbon dioxide from gas streams, but it's corrosive and toxic in very small amounts. Ascarite II is a very efficient CO₂ absorbent, but it's basically nonfibrous asbestos covered with sodium hydroxide.

Potassium superoxide is an interesting possibility for spacecraft and submarine CO₂ scrubbing, since it regenerates oxygen as it reacts with carbon dioxide:

4 KO₂(s) + 2 CO₂(g) = 2 K₂CO₃(s) + 3 O₂(g) But it isn't common, and it is quite toxic.

Calcium hydroxide (mixed with a small amount of sodium and potassium hydroxides) is used in most underwater rebreathers. The reaction between the hydroxides and CO₂ is exothermic, and divers can tell from the warmth of the scrubber canister that the absorption reaction is working. Failure of the canister lid can give the diver a mouthful of hydroxides- called a caustic cocktail in diving circles. It's apparently a memorable experience.

You can learn more about the construction and chemistry of rebreathers here.

The U. S. Department of Energy maintains a site on CO₂ removal technologies being considered to reduce global carbon emissions.

Author: Fred Senese senese@antoine.frostburg.edu

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/inorganic/faq/co2-scrubbers.shtml